There is growing interest in incorporating electronics into everyday objects, including clothing and textile objects which are expected to be pliable, stretchable and soft. Soft electronics would be able to seamlessly interface with the human body, opening up many new applications for wearables, medical devices and the prospect of conformal robotics or ‘soft machines’ which could more safely interact with humans or delicate items (see, for example, Dickey, ACS Appl Mater Interfaces. 2014 Nov. 12; 6(21): 18369-18379).
Many non-traditional manufacturing methods are being considered to fabricate these soft electronic devices, notably 3D printing. However, a major stumbling block is the lack of a high conductivity and easily processed stretchable conductor.
Many attempts at stretchable conductors have been tried. One of the most successful has been microfluidic channels filled with room temperature liquid metal. Functional devices are created by etching micrometer channels into polydimethylsiloxane (PDMS), sealing them over and then injecting metals alloy into the channels to create conductive paths. These alloys can have a melting point as low as −19° C. and so remain fluid under normal conditions. Because the metal conductors are fluid, they can be deformed to an extent limited only by the material creating the channels containing them and recover fully. Further, their change in resistance is a purely mechanical function of the wire length and cross section and so is linear. This affords a major advantage of allowing conductive pathways to also act as sensors.
However. manufacturing devices with these microfluidic channels is very challenging. PDMS is the preferred substrate, but creating a proper seal around the channels is expensive, requiring exposure of the channel containing layer to oxygen plasma in order to adhere a cap layer to enclose the channel. Once constructed, the channels must be filled via a two syringe system, where one syringe injects the liquid alloy and another evacuates the air already present. Failure rates during construction are very high. Anecdotally it's been reported that only about one in twenty fabrication attempts succeeds. Thus, the need exist for other “liquid” metals.